FIG. 9 shows a conventional arrangement wherein base driving circuit DR controls power transistor TR, which is used as a switching power device. TR includes NPN power transistors connected to each other in a Darlington arrangement. Driving power source 3 supplies power to base driving circuit DR. Si1 and Si2 denote input terminals of the base driving circuit to which driving signal S is applied. Driving signal S controls the on-off states of power transistor TR. In this arrangement, the power transistor is on only when photo-coupler PC1 is turned on, and it is off otherwise.
FIG. 8 shows an exemplary inverter circuit which comprises a multiplicity of base driving circuits each identical to base driving circuit DR of FIG. 9. It also comprises power transistors TR1, TR2, TR3, TR4, TR5 and TR6 each identical to power transistor TR of FIG. 9.
In FIG. 8, TR1-TR6 are power transistors of an NPN type which are connected in a bridge arrangement to form a three-phase inverter for driving induction motor IM. This inverter converts DC electric power or voltage into AC voltages of three phases, which are denoted U, V, and W. The DC power is applied between main DC electrodes denoted positive line (+) LP and negative line (-) LN. The AC voltage of each phase is individually supplied to IM from a connection point (or midpoint) between upper and lower arms in the bridge arrangement. As shown in FIG. 8, transistors TR1, TR3 and TR5 are in the upper arm. Because the negative electrodes of their corresponding driving power supplies 3 are respectively connected to AC output lines of different electric potentials due to the different phases involved, driving power supplies 3 are required to be insulated from one another. By contrast, only one driving power supply 3 may be required for transistors TR2, TR4 and TR6 in the lower arm, because the negative electrodes of their corresponding driving power supplies 3 are all connected to the negative line (-) LN of the main DC power supply which has a fixed electric potential. This being so, this inverter circuit in a three-phase bridge arrangement may comprise a minimum of four driving power supplies--three for the upper arm and one for the lower arm.
Turn now to FIG. 7 which illustrates different situations where short-circuits occur and the causes associated therewith. These short circuits, which are characterized by their short-circuit paths, are, for example, an arm short-circuit, a series arm short-circuit, an output short-circuit and ground.
FIG. 6 shows the so-called "overload safe operation area (OLSOA)" of a power transistor in its characteristic plot of collector-emitter voltage (V.sub.CE) vs. collector current (I.sub.cp). The OLSOA is important when any one of the aforementioned short-circuit situations arises. In one such situation, an overcurrent flows through power transistor TR, and the power transistor breaks down once the locus or the value of I.sub.cp falls outside of the OLSOA. This being so, the power transistor has to be immediately turned off upon detecting the overcurrent. This can be accomplished by a circuit shown in FIG. 5.
FIG. 5 illustrates a basic, base driving circuit with an overcurrent protection capability. In FIG. 5, the broken line encloses overcurrent protection circuit OC. The power-transistor protection using the circuit of FIG. 5 is realized as follows:
(1) Driving signal S is applied to terminals Si and O for turning on power transistor TR. The driving signal charges capacitor C1 through resistor R1, thereby simultaneously turning on power transistor TR through resistor R2.
(2) Power transistor TR, in a normal operation, has a much lower saturated collector-emitter voltage V.sub.CE (sat) than the avalanche voltage of zener diode ZD. This being so, capacitor C1 discharges toward collector C of power transistor TR through diode D2.
(3) In a short-circuit situation, an excessive current starts to flow through power transistor TR, thereby raising V.sub.CE (sat) of TR above the avalanche voltage of zener diode ZD. As a result, capacitor Cl is prevented from discharging toward power transistor TR.
(4) Consequently, the voltage of capacitor Cl is raised above the avalanche voltage of zener diode ZD, thereby turning on auxiliary transistor Q2 and lowering the base-emitter voltage of power transistor TR to the saturated collector-emitter voltage V.sub.CE (sat) This results in turning power transistor TR off.
FIG. 3 illustrates a conventional base driving (in this instance, gate driving) circuit with an overcurrent protection capability, wherein an insulated gate bipolar transistor (IGBT), which is equivalent to power transistor TR in FIG. 5, is used as a switching power device. In FIG. 3, the broken line encloses an overcurrent protection circuit denoted OC, and the rest of FIG. 3 describes base driving circuit denoted DR. The basic function of the circuit of FIG. 3 is same as that of FIG. 5. However, driving signal S in FIG. 3 is provided through photo-coupler PC1 identical to that in FIG. 9, and base driving circuit DR in FIG. 3 is relatively complex, compared with that in FIG. 5.
The circuit of FIG. 3, however, lacks a way of indicating the activity of overcurrent protection circuit OC to the outside thereof. Thus, this circuit is normally provisioned with alarming circuit ALMG as shown in FIG. 4. For the sake of convenience, the overcurrent protection circuit in FIG. 4, supported by the alarm circuit, is hereinafter referred to as overcurrent protection circuit with alarming circuit OC1.
In a conventional three-phase inverter circuit, alarm signal ALM in FIG. 4 is applied to a driving signal generating circuit for cutting off driving signal S so as to simultaneously turn off the six switching power devices. These switching power devices are thus protected, notwithstanding that an overcurrent flows through only a subset of them.
The conventional overcurrent protection circuit described above has the following shortcomings. When the base driving circuit with the alarm capability in FIG. 4 is used in the three phase inverter circuit in FIG. 8, the leads for transporting, to a controlling apparatus, alarm signals ALM for individual power transistors need to be insulated from one another. This controlling apparatus (for example, a microcomputer) controls the driving signals to the six transistor elements. The need of insulation stems from the fact that the electric potentials of driving power supplies 3 for individual transistors TR1, TR3 and TR5 in the upper arm are different not only from those for transistors TR2, TR4 and TR6 in the lower arm, but also from one another. As a result, a total of at least four photocouplers, each identical to PC2 of FIG. 4, are needed for the alarming purposes in the inverter circuit. This arrangement undesirably calls for a complex circuit with a significant number of components. Accordingly, it is desirable to simplify the protection circuit to reduce the number of components.